Attacking Species and Types of Organic Reactions
Let us first study about some species that are responsible for a number of reactions.
Electrophiles are those species that have electron deficient atom in them. Some examples of electrophiles are :
H3O+, Cl+, NO2+, R+ etc.
Electrophiles need not be positively charged species. Some electrically neutral molecules can also act as electrophiles. Examples are :
Some electrically neutral molecules that act as electrophile
BF3, AlCl3, FeCl3 etc.
Why do AlCl3, BF3, and FeCl3 act as electrophiles ?
Answer : Let us take an example of AlCl3 and see why it acts as an electrophile. It's structure is given below :
In AlCl3 molecule, all the chlorine atoms have complete octets. Aluminium, however, has only sixtet and not octet. In other words, aluminium is electron deficient. Due to this reason aluminium tries to gain electrons, and hence acts as an electrophile. Similarly, BF3 and FeCl3 also act as electrophiles because B and Fe are electron deficient in these molecules.
Due to the deficiency of electrons, electrophiles have a strong tendency to attract electrons from other species and hence behave as Lewis acids.
Nucleophiles are electron rich species. Some examples are :
Cl−, Br−, I−, OH−, OR−, NH2−, CN−, RCOO− etc.
Some electrically neutral species can also act as nucleophiles. For example, H2O can act as nucleophile because of the presence of two lone pairs on oxygen atom in water molecules (). Some other examples are :
Some electrically neutral molecules that act as nucleophiles
NH3, R−NH2, R−OH, ROR' etc.
Due to the presence of at least one unshared pair of electrons, nucleophiles have a strong tendency to donate electrons to electron deficient species. Hence, they behave as Lewis bases.
Free radicals are those species that have unpaired electron. They are also attacking species.
Types of Organic Reactions
There are many types of organic reactions. Most important being :
- Substitution Reactions
- Addition Reactions
- Elimination Reactions
- Rearrangement Reactions
Let us study them one by one :
In substitution reactions, an atom or a group of atoms in a molecule is replaced by some other species. Depending upon the type of attacking species, the substitution reactions can be termed as :
- Nucleophilic substitution reactions
- Electrophilic substitution reactions
- Free radical substitution reactions
Nucleophilic substitution reaction
In nucleophilic substitution reactions, the attacking species is a nucleophile. In such reactions, a weaker nucleophile is replaced by a stronger nucleophile. Nucleophilic substitution reactions are prevalent in alkyl halides.
Electrophilic substitution reactions
In electrophilic substitution reactions, the attacking species is an electrophile. Aromatic compounds such as arenes usually undergo electrophilic substitution reactions.
In the above reaction dichlorine reacts with anhydrous AlCl3 to give chloronium ion (Cl+) which replaces one of the hydrogen atoms of benzene ring.
Free radical substitution reactions
In free radical substitution reactions, its the free radical that acts as attacking species.
You may not be able to grasp the concept just by looking at the above example. Let us dig deeper and see how methyl cholride is formed.
Dichlorine in the presence of light undergoes homolytic fission and gives chlorine free radicals.
Chlorine free radicals thus formed are responsible for the following reactions.
In addition reactions, two or more molecules combine to form one large molecule. Addition reactions are typically seen in compounds containing multiple bonds. Depending upon the type of attacking species that initiates the reactions, they are of following types :
- Nucleophilic addition reactions
- Electrophilic addition reactions
- Free radical addition reactions
Nucleophilic addition reactions
Nucleophilic addition reactions are those reactions that are initiated by nucleophiles. These reactions are prevalent in aldehydes and ketones. The addition of HCN to ethanal is an example of nucleophilic addition reaction.
Notice that in the above reaction, there is an addition of both nucleophile (CN−) and electrophile (H+). Even so, the reaction is termed as nucleophilic addition reaction. The reason for this is that the nucleophile is responsible for the above reaction to occur. A deeper reason is that when a nucleophile attacks in such reactions, a new bond between the nucleophile and carbon is formed which is the rate determining step.
Electrophilic addition reactions
Electrophilic addition reactions are those reactions that are initiated by electrophiles. These reactions are prevalent in alkenes and alkynes. Addition of HBr to propene is an example of electrophilic addition reaction.
Again, there is an addition of both electrophile and nucleophile but the reaction is electrophilic addition reaction. What do you think is the reason?
Free radical addition reactions
Free radical addition reactions are initiated by free radicals.
Let us study its mechanism :
The peroxide used in the above reaction is first cleaved to give alkoxy free radicals.
The produced alkoxy free radical is responsible for the formation of bromine free radical.
Bromine free radical thus formed causes the following reaction to occur. This reaction being slow is the rate determining step.
Another molecule of HBr reacts with isopropyl free radical to give the desired product.
Elimination reactions involve the removal of two substituents from a molecule that results in the formation of multiple bonds. Depending upon the relative positions of eliminated substituents, these reactions are classified as :
- α-elimination : When both the substituents are removed from the same atom.
- β-elimination : When the substituents are removed from adjacent atoms.
- γ-elimination : When the relative position of removed substituents is 1,3.
Example : The following reaction is an example of β-elimination :
Some elimination reactions have their specific names. Examples are :
- Dehydrohalogenation : Dehydrohalogenation reactions are those elimination reactions in which the eliminated substituents are hydrogen and halogens.
- Dehalogenation : In dehalogenation reactions, both the substituents are halogens.
- Dehydration : Elimination of a water molecule (H2O) is known as dehydration.
- Dehydrogenation : In dehydrogenation, both the removed substituents are hydrogens.
In rearrangement reactions, substituents migrate from one atom to another within the same molecule. The following reaction is an example of rearrangement reaction.
Explanation : When 2,2-dimethylpropan-1-ol reacts with conc. H2SO4, the hydroxyl group attached to primary carbon gets removed leading to the formation of primary (1°) carbocation. Since primary carbocation is least stable among all carbocations (why?), one of the alkyl groups attached to the tertiary carbon migrates to the positively charged carbon leading to the formation of tertiary (3°) carbocation.
Tertiary carbocation thus formed loses a proton to give 2-Methylbut-2-ene.
Some Other Important Reactions
In condensation reaction, two molecules combine together to form a large molecule with the loss of small molecules such as water, HCl.
Reactions that involve transformation of one isomer into another keeping the molecular formulae as well as the carbon skeletons of the reactants and the products intact are called isomerisation reactions.
The process of converting small molecules, called monomers, into a polymer chain is called polymerisation.
Which type of reaction is this :
Which type of reaction is this :